The present invention relates to a current resonance type DC-DC converter (direct current to direct current) and a method for operating a current resonance type DC-DC converter. Specifically, the current resonance type DC-DC converter includes a switching circuit and a resonance circuit that are connected to a primary winding of a transformer. The switching circuit has at least a pair of first switching elements, which are alternatively turned ON and OFF, and is configured with a push-pull type circuit or a bridge type circuit. The resonance circuit generates resonance current at the primary winding of the transformer.
A DC-DC converter that is used in a switching power supply device is disclosed in Japanese Patent Publication Number H08-289540. In this DC-DC converter, a first capacitor for smoothing (smoothing capacitor) is connected between a pair of power supply terminals. Specifically, the smoothing (first) capacitor works as a DC power source of this circuit. Further, a series circuit of first and second switches is connected in parallel to the smoothing capacitor. In this case, the first and second switches are configured with a Metal Oxide Semiconductor (MOS) Field Effect Transistor (FET). Further, the first and second switches work as a control switch, which corresponds to the original functions of a FET, and a diode, which is connected in an antiparallel connection thereto.
Further, a series circuit of a primary winding and a resonance capacitor (an output resonance circuit) are connected between a connection point between the first and the second switches and a lower end of the smoothing capacitor (i.e., a source of the second switch). The primary winding has inductance for resonance. Further, the primary winding of a transformer has excitation inductance in addition to inductance that is configured with leakage inductance. Specifically, the excitation inductance is equivalently connected to the primary winding in parallel. On the other hand, a secondary winding of the transformer is divided into first and second windings by a center tap. The ends of the secondary winding are connected to one terminal of an output smoothing capacitor through third and fourth diodes, respectively. The center tap of the secondary winding is connected to another terminal of the output smoothing capacitor. Further, a pair of output terminals that are connected to a load (not shown) are connected to the output smoothing capacitor. In addition, a control circuit, which alternately turns ON and OFF the first and the second switches, is configured so that an output voltage is controlled to be a stable voltage by changing the ON and OFF frequencies of the first and the second switches according to a change of an input voltage or an output voltage. Specifically, the input voltage corresponds to a charging voltage of the smoothing capacitor and the output voltage corresponds to a charging voltage of the output smoothing capacitor.
In the case in which the smoothing capacitor has been charged in the DC-DC converter, when the first switch is turned ON, an electric current flows in a series resonance circuit by series resonance. The series resonance circuit is configured with a closed circuit that has the smoothing capacitor, the first switch, the primary winding and the resonance capacitor. Similarly, when the second switch is in an ON period, an electric current flows in a series resonance circuit by series resonance. The series resonance circuit is configured with a closed circuit that has the resonance capacitor, the primary winding and the second switch. As a result, because the series resonance circuit that is configured with the leakage inductance of the primary winding and the capacitor is driven by the ON and OFF operations of the first and second switches, an output power that corresponds to the electric current (electric power) based on the series resonance is obtained at the secondary winding of the transformer. The DC-DC converter that has this configuration corresponds to an LLC current resonance type converter. Within a range of frequency in which the output power greatly changes when the frequency is changed, the control circuit controls the output voltage to be stable by changing the ON and OFF frequencies of the first and the second switches (by a frequency control).
The conventional DC-DC converter described above, however, still has some problems to be solved. That is, it is preferred that the conventional DC-DC converter has a configuration in which the output voltage can be greatly changed when the frequency is changed by increasing an λ value of the output resonance circuit, which is configured with the primary winding of the transformer and the resonance capacitor, to perform the frequency control by the control circuit as explained above. Note that, in regards to the λ value, λ=Lr/Lm is satisfied, here Lr corresponds to a value of the leakage inductance and Lm corresponds to a value of the excitation inductance. Here, the value of the leakage inductance, which configures the resonance circuit, needs to become large in order to make the λ value large. However, when the value of the leakage inductance becomes large, a large voltage is applied to the transformer. Therefore, a magnetic path cross-sectional area of the transformer needs to be increased. Further, the number of turns of the primary winding and a secondary winding of the transformer need to be increased. As a result, a volume of the transformer is increased. This means that the transformer increases in size. In addition, the resonance circuit can be configured by using an independent inductor instead of the leakage inductance. However, even though this configuration is used, when an inductance value of this inductor becomes large, a large voltage is applied to the inductor and a magnetic path cross-sectional area of the inductor needs to be increased. Further, the number of turns of the inductor needs to be increased. Therefore, the volume of the inductor increases. As a result, the same problem, i.e., that the volume of the DC-DC converter increases, occurs in either configuration discussed above.
On the other hand, it is possible to control the output voltage by changing a duty ratio of the first and the second switches (PWM (pulse width modulation) control). However, when the duty ratio is decreased by this control, a voltage between a drain and a source of the first and second switches may oscillate. In this case, a problem occurs in that a zero volt switch operation becomes difficult.
The present invention aims to solve these problems. Thus, an object of the present invention is to provide a current resonance type DC-DC converter that can perform a zero volt switch operation and that can avoid increasing a volume of the converter.